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Register a new userMagnetic behavior of metallic kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12
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E. V. Sampathkumaran Professor
Publication date
2017
fields
Physics
and research's language is
English
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We report magnetic behavior of two intermetallics-based kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12, crystallizing in the Gd3Ru4Al12-type hexagonal crystal structure, by measurements in the range 1.8-300 K with bulk experimental techniques (ac and dc magnetization, heat-capacity and magnetoresistance). The main finding is that the Tb compound, known to order antiferromagnetically below (T_N=) 22 K, shows glassy characteristics at lower temperatures (<15K), thus characterizing this compound as a re-entrant spin-glass. The data reveal that glassy phase is quite complex and is of a cluster type. Since the glassy behavior was not seen for the Gd analogue in the past literature, this finding for the Tb compound emphasizes that this kagome family could provide an opportunity to explore the role of higher order (such as quadrupole) in bringing out magnetic frustration. Additional findings reported here for this compound are: (i) The temperature dependence of magnetic susceptibility and electrical resistivity in the range 12 - 20 K are found to be hysteretic leading to a magnetic phase in this temperature range, mimicking disorder-induced first-order magnetic phase-transition. (ii) Features attributable to an interesting magnetic phase co-existence phenomenon in the magnetoresistance in zero field, after cycling across metamagnetic transition fields, are observed. With respect to the Er compound, we do not find any evidence for long-range magnetic ordering down to 2 K, but this appears to be on the verge of magnetic order at 2 K.
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We present the results of neutron diffraction studies on polycrystals of a metallic kagome lattice, Tb3Ru4Al12, reported recently to undergo reentrant magnetism, with the onset of long range antiferromagnetic order below (TN=) 22 K and spin-glass features below about 17 K. The present results reveal long-range antiferromagnetic order of an incommensurate type with the moments oriented along c-axis at all temperatures below TN. There are however notable changes in the T dependence of propagation vector along b-axis across 17 K. An observation of interest is that there is no decrease of intensity of magnetic Bragg peaks on entrance into the glassy phase (that is, below 17 K). This finding suggests that the magnetism of this compound is an exotic one and we wonder whether this compound is an example for dynamical spin-glass freezing phenomenon, as a consequence of geometrical frustration.
The magnetic behavior of the compound, Gd3Ru4Al12, which has been reported to crystallize in a hexagonal structure about two decades ago, had not been investigated in the past literature despite interesting structural features (that is, magnetic layers and triangles as well as Kagome-lattice features favouring frustrated magnetism) characterizing this compound. We report here the results of magnetization, heat-capacity, and magnetoresistance studies in the temperature (T) range 1.8-300 K. The results establish that there is a long-range magnetic order of an antiferromagnetic type below (TN= ) 18.5 K, despite a much large value (about 80 K) of paramagnetic Curie temperature with a positive sign characteristic of ferromagnetic interaction. We attribute this to geometric frustration. The most interesting finding is that there is an additional magnetic anomaly below about 55 K before the onset of long range order in the magnetic susceptibility data. Concurrent with this observation, the sign of isothermal entropy change remains positive above TN with a broad peak above TN. This observation indicates the presence of ferromagnetic clusters before the onset of long range magnetic order. Thus, this compound may serve as an example for a situation in which magnetic frustration due to geometrical reasons is faced by competition with such precursor effects. There is also a reversal of the sign of entropy-change in the curves for lower final fields (for H less than 30 kOe) on entering into magnetically ordered state consistent with the entrance into antiferromagnetic state. The magnetoresistance behavior is consistent with above conclusions.
A previously unreported Pb-based perovskite PbMoO$_3$ is obtained by high-pressure and high-temperature synthesis. This material crystallizes in the $Pmbar{3}m$ cubic structure at room temperature, making it distinct from typical Pb-based perovskite oxides with a structural distortion. PbMoO$_3$ exhibits a metallic behavior down to 0.1 K with an unusual $T$-sub linear dependence of the electrical resistivity. Moreover, a large specific heat is observed at low temperatures accompanied by a peak in $C_P/T^3$ around 10 K, in marked contrast to the isostructural metallic system SrMoO$_3$. These transport and thermal properties for PbMoO$_3$, taking into account anomalously large Pb atomic displacements detected through diffraction experiments, are attributed to a low-energy vibrational mode, associated with incoherent off-centering of lone pair Pb$^{2+}$ cations. We discuss the unusual behavior of the electrical resistivity in terms of a polaron-like conduction, mediated by the strong coupling between conduction electrons and optical phonons of the local low-energy vibrational mode.
Detailed electronic structure calculations of picene clusters doped by potassium modeling the crystalline K3picene structure show that while two electrons are completely transferred from potassium atoms to the LUMO of pristine picene, the third one remains closely attached to both material components. Multiconfigurational analysis is necessary to show that many structures of almost degenerate total energies compete to define the cluster ground state. Our results prove that the 4s orbital of potassium should be included in any interaction model describing the material. We propose a quarter filled two orbital model as the most simple model capable of describing the electronic structure of K-intercalated picene. Precise solutions obtained by a development of Lanczos method show low energy electronic excitations involving orbitals located at different positions. Consequently, metallic transport is possible in spite of the clear dominance of interaction over hopping.
We have investigated the magnetic behavior of the nanocrystalline form of a well-known Laves phase compound, ErCo2 - the bulk form of which has been known to undergo an interesting first-order ferrimagnetic ordering near 32 K - synthesized by high-energy ball-milling. It is found that, in these nanocrystallites, Co exhibits ferromagnetic order at room temperature as inferred from the magnetization data. However, the magnetic transition temperature for Er sublattice remains essentially unaffected as though the (Er)4f-Co(3d) coupling is weak on Er magnetism. The net magnetic moment as measured at high fields, sat at 120 kOe, is significantly reduced with respect to that for the bulk in the ferrimagnetically ordered state and possible reasons are outlined. We have also compared the magnetocaloric behavior for the bulk and the nano particles.
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